This invention relates to protective coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method of stabilizing the microstructure of a thermal barrier coating (TBC) with alumina precipitates in order to inhibit degradation of the thermal insulating properties of the TBC during high temperature excursions.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.
Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques. such as air plasma spraying (APS) deposit TBC material in the form of molten xe2x80x9csplats,xe2x80x9d resulting in a TBC characterized by a degree of inhomogeneity and porosity that reduces heat transfer through the TBC.
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC maintains a low thermal conductivity throughout the life of the component. However, the thermal conductivities of TBC materials such as YSZ have been observed to increase by 30% or more over time when subjected to the operating environment of a gas turbine engine. This increase has been associated with coarsening of the zirconia-based microstructure through grain and pore growth and grain boundary creep. To compensate for this phenomenon, TBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary.
Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
The present invention generally provides a thermal barrier coating (TBC) for a component intended for use in a hostile environment, such as the superalloy. turbine, combustor and augmentor components of a gas turbine engine. TBC""s of this invention have microstructures that are less susceptible to grain sintering and pore coarsening during high temperature excursions, all of which lead to densification of the TBC. Improvements obtained by this invention can be realized with TBC""s deposited to have a columnar grain structure, such as those deposited by EBPVD and other PVD techniques, as well as noncolumnar TBC""s that are inhomogeneous and porous as a result of being deposited by plasma spray techniques, such as air plasma spraying (APS).
Typical YSZ source materials available for use in deposition processes often contain more than 0.1 mole percent of impurities, such as oxides of silicon, titanium, iron, nickel, sodium, lithium, copper, manganese and potassium. According to the invention, several of these impurities (particularly silica) in aggregate amounts of as little as 0.02 mole percent can be sufficient to form amorphous phases with low glass transition temperatures or phase/surface modifications that promote surface diffusion. These undesirable phase types decorate nearly all of the grain boundaries and the splat boundaries (for plasma-sprayed TBC) or column boundaries (for PVD TBC) of a TBC. At such levels, these phase types are believed to sufficiently wet the boundaries to allow or promote grain sintering and coarsening and/or surface diffusion kinetics that lead to densification of the TBC, the end result of which is an increase in the thermal conductivity of the TBC. Reducing impurity levels in YSZ source materials to eliminate this densification effect in the TBC can be prohibitively expensive.
As a solution, the invention employs small amounts of alumina precipitates (crystalline structures) dispersed throughout the grain boundaries of the TBC to getter impurities, and particularly the oxide impurities noted above. As used herein, the term xe2x80x9cgetterxe2x80x9d includes various mechanisms by which sintering that would be enhanced by the presence of impurities is neutralized (negated) or at least minimized. Examples of gettering mechanisms include (a) the formation of alumina-containing crystalline compounds such as mullite (3Al2O3.2SiO2) alumina titanate (Al2O3.TiO2) and/or Al2O3.MnO2, and (b) the formation of solid solutions with various compounds, including FeO, Fe2O3, etc. As a result of being insoluble in zirconia, these alumina-based reaction products form precipitates that can advantageously reduce grain boundary mobility of the YSZ TBC. As such, the alumina precipitates inhibit densification. and the associated increase in thermal conductivity caused by grain sintering and coarsening and/or surface diffusion kinetics that are promoted by the presence of impurities. Another benefit is that, if the alumina precipitates are sufficiently fine, such as on the order of about 2 to 500 nm, the precipitates are able to pin the grain, pore and/or feathery substructure boundaries within the TBC. In doing so, the tendency is reduced for the microstructure of the TBC to sinter, coarsen and undergo pore redistribution (as used herein, when smaller pores coalesce or coarsen to form larger pores) during high temperature exposures, such as temperatures in excess of 1000xc2x0 C. found within the hot gas path of a gas turbine engine.
According to the invention, incorporating relatively low levels of alumina precipitates serves to reduce or eliminate undesirable impurity effects, while higher levels provide the additional benefit of further stabilizing the YSZ grain structures against coarsening attributable to surface diffusion and grain boundary motion. Accordingly, by providing a small but sufficient amount of fine alumina precipitates within a TBC microstructure, the TBC can be subsequently heated to temperatures in excess of 1200xc2x0 C. without densification and an associated increase in thermal conductivity. As a result, components can be designed for thinner TBC and/or, where applicable, lower cooling air flow rates, which reduces processing and material costs and promotes component life and engine efficiency.
Other objects and advantages of this invention will be better appreciated from the following detailed description.